A number of optical methods have been developed over the years to attempt to reduce or eliminate the progression of myopia. These methods attempt to extend their associated vision correction device to one which is a vision (i.e. refractive error) treatment device. In this document, “vision correction devices” that employ optical methods for eliminating or reducing the progression of refractive errors will be called “vision treatment devices”.
One much-attempted optical method is “under-correction” in which the wearer is prescribed an optical power less than necessary to fully correct his refractive error. Since the position of the visual image is not relocated to the retina, constant blurred vision is an implicit and undesirable consequence of the under-correction method. Due to the constant blurred vision, compliance of the wearer to maintain this method of treatment is likely to be poor.
Other optical methods employ bifocal or progressive aspheric lens spectacles or bifocal contact lenses as potential strategies for retarding the progression of myopia. For example, U.S. Pat. No. 6,343,861 to Kris discloses the use of a progressive ophthalmic lens designed to reduce the rate of juvenile myopia progression. U.S. Pat. No. 6,752,499 to Aller discloses a method for treating myopia progression in patients who also exhibit near point esophoria by selectively prescribing bifocal contact lenses. WO 2006/004440 to Phillips discloses the use of bifocal contact lenses with various arrangements of vision correction area and myopic defocus area to counter progression of myopia. US 2006/0082729 to describes the use of Fresnel-type lenses to produce two refractive powers (i.e. a bifocal effect) to treat the progression of myopia.
However, studies on the efficacy of methods that employ bifocal devices generally show only limited efficacy. In the case of bifocal or progressive spectacles, compliance of the wearer to always look through the near addition portion of the spectacles for near work cannot be guaranteed. And this is particularly so when dealing with children. The bifocal contact lenses that have been used to date have been simultaneous vision bifocals—i.e. both the distance and the near images are produced in the eye simultaneously. Such bifocals degrade the overall retinal image quality and are known to produce undesirable visual problems such as haloes, glare and ghosting.
Yet other optical methods seek to reduce the progression of myopia by manipulating the aberrations in the visual image introduced to the eye. “Aberrations” refers to the optical performance characteristic of an optical lens or system that relates to how poorly a visual image is produced by that lens or system. When a visual image is formed perfectly sharply or distinctly (relative to the limits of diffraction which is an ultimate physical limit of how sharply a focus can be produced) and in the required location in space, the image is said to be free of aberrations. With departure from this perfect state, the visual image is said to suffer from aberrations. It is thought by some that certain aberrations can influence the progression of myopia.
A few optical methods relating to the manipulation of on-axis (or axial) optical performance of the vision device, or its on-axis aberrations, have been developed for the treatment of myopia progression. “On-axis” or “axial” is a description of the direction of light relative to the direction of vision. The most visually acute point on the retina (the light-sensing layer of the eye) is the fovea. This is a small area on the retina at which the light-sensitive photo-receptors are at their highest concentration. When an individual needs to critically view a visual object, he does so by changing the direction of gaze (by rotation of the head or rolling of the eyeball up/down, left/right) so that the point of interest in the image is placed over the fovea of the eye. This process of aligning the eye's most sensitive point to the visual object of interest is called “fixation”. “On-axis” or “axial” refers to when light arriving to the eye originates from the point in the visual object which is imaged on to the fovea, i.e. the point of fixation, or in the straight-ahead direction. In this situation, the light rays arriving to the eye are approximately parallel to the axis of the eye.
For example, U.S. Pat. No. 6,045,578 to Collins discloses a method of treatment and prevention of myopia by inducing positive spherical aberration (an on-axis aberration) in the myopic eye. US 2003/0058404 to Thorn describes a method of measuring and correcting the wave-front aberrations of parallel light rays entering the eye in order to prevent or retard the progression of myopia. US 2004/0237971 to Radhakrishnan describes the manipulation of aberrations to control the relative position of modulation transfer function peaks in order to retard or abate the progression of myopia.
The general efficacy of the manipulation of on-axis optical methods is yet to be definitively proven. In at least some of the on-axis optical methods described (for example, the induction of positive spherical aberration), the method necessarily implicitly degrades the visual acuity and could lead to poor compliance on the part of the patient and would therefore suffer the same disadvantages as under-correction methods.
In contrast to these optical methods which deal with the manipulation and control of optical focus and aberrations in the straight-ahead, on-axis direction, the disclosure of U.S. Pat. No. 7,025,460 demonstrated that myopia progression is controlled by the off-axis optical characteristics. Converse to on-axis, “off-axis” refers to when light is arriving to the eye from a direction other than straight-ahead; i.e. the image points corresponding to the off-axis object points lie away from the fovea. Off-axis visual direction is also referred to as “peripheral vision” and the object points (points in the visual scenery) belonging to the off-axis direction the “peripheral visual field” or simply “peripheral field”. When light arrives to the eye from an off-axis direction, it creates an angle with the straight-ahead direction of view of the eye. This angle is sometimes called the “field angle”.
U.S. Pat. No. 7,025,460 describes a set of experiments and observations that demonstrate that retarding or eliminating eye-growth that leads to progression of myopia may best be effected by controlling the peripheral visual image. From those observations, U.S. Pat. No. 7,025,460 teaches an optical method for treating the progression of myopia by manipulating the positions of peripheral (i.e. off-axis) visual image points, or the relative curvature of field of the visual image.
It should be mentioned that curvature of field is the type of off-axis optical aberration that relates to the antero-posterior position (i.e. whether further in front of, or further behind) of the peripheral image points (of the visual image) relative to the preferred image-receiving surface (which in the eye, is the retina). Curvature of field differs fundamentally from spherical aberration (for example as taught by U.S. Pat. No. 6,045,578 and US 2003/0058404). Spherical aberration is the optical aberration that describes whether light rays, all from the same straight-ahead (along the visual axis) direction, but passes through different points on the pupil of the eye, are focused to the same image point. Thus spherical aberration relates to how well (or sharply) an image point from its corresponding object point from the straight-ahead direction is focused whereas curvature of field relates to where in space (antero-posteriorly or forward-backward position-wise) image points from many different directions (i.e. from different field angles) in the visual scenery is positioned regardless of how sharply they are focused. The set of all such image points can be described as an image surface. So curvature of field relates to the shape and position of the image surface.
In comparison, it should also be noted that the bifocal optical methods (for example, as taught by U.S. Pat. No. 6,752,499) seeks to create two image points for each visual object point (this is a feature of simultaneouos vision bifocal contact lenses). Thus, ‘double image’ is implicitly created—one from the near focus zone and one from the distance focus zone of the bifocal. In comparison, the control of curvature of field creates only a single image point for each visual object point but governs the antero-posterior position of the image point relative to the image-receiving surface.
One aspect of U.S. Pat. No. 7,025,460 is a method of designing a vision treatment device (for example, contact lens, spectacle lens, corneal inlay or onlay, etc.) to be worn by a wearer that will manipulate the positions of the peripheral image points (that is, manipulate the relative curvature of field) in such a way so as to produce stimuli to reduce or eliminate the progression of myopia in the wearer while simultaneously maintaining the position of the on-axis visual image point on the retina/fovea so as to maintain good visual acuity for the wearer.
Designing a vision treatment device according to the teachings of U.S. Pat. No. 7,025,460, depending on the exact shape of the image surface (i.e. relative curvature of field) to be presented to the eye, may require some trade-off between the manipulation of relative curvature of field and the amount of other optical aberrations that results. Since most conventional optical vision correction devices have typically only two (one anterior, one posterior) optical surfaces, when the lens is designed to manipulate relative curvature of field, due to the limited number of design parameters (e.g. lens surface shape, refractive index of material, lens thickness, distance from the pupil, etc) limiting the degrees of freedom in optical design, some other optical aberrations may be concomitantly introduced or altered. Such other optical aberrations (i.e. aberrations other than curvature of field), may be described according to the von Seidel classification of aberrations as is well-known to those skilled in optics and lens design. These include spherical aberration (which has already been described above) as well as coma, oblique astigmatism and distortion. Throughout this document, we will refer to these as the “other optical aberrations”.
One other category of aberration is the chromatic aberration. This aberration is related to how light of different colors (wavelengths) creates different focal positions and does not impact on the concept and applicability of the present invention.
In terms of other optical aberrations, vision optical devices can be divided roughly into two groups according to whether they remain substantially approximately aligned with the direction of view of the eye with different directions of gaze of the eye.
Vision correction devices of the first group can be called “centered” vision correction devices and include contact lenses, intra-ocular lenses, on-lays, in-lays and anterior chamber lenses. The optical axis of these vision correction devices remains substantially approximately aligned with the direction of view of the eye regardless of its direction of gaze. For the centered vision correction devices, light from the on-axis visual object always passes approximately through the central region of the device on its way to the fovea after passing through the pupil of the eye.
Vision correction devices of the second group can be called “decenterable” vision correction devices and include spectacles and translating-type (e.g. translating bifocal) contact lenses. Devices in this group do not remain aligned with the direction of view of the eye depending on the direction of gaze of the eye.
For centered vision correction devices, undesirable other optical aberrations may arise through the portion of the optical device that corresponds to the off-axis directions or peripheral fields in the manner described above. This also applies to the decenterable vision correction devices when the eye is in the straight-ahead gaze position with the direction of vision passing through or near the optical center of the device.
For decenterable vision correction devices, the other optical aberrations produced by the periphery of the optical device may also impact foveal vision. This occurs when the eye is not in straight-ahead gaze. When the eye is in the straight-ahead gaze, the line of sight of the eye passes through the device at what is called the “distance visual point”. Typically, except for certain special applications, for best visual performance, the distance visual point is placed near or at the optical center of the device. When the eye is rotated away from straight-ahead gaze, it will no longer be looking through the center of the device. In those “eccentric” directions (i.e. a direction of gaze not in the straight-ahead direction) of gaze, the image produced on the foveal region will be constructed from light rays that pass through a peripheral portion of the device. Such an image will incur and suffer from the other optical aberrations produced by the peripheral portion of the device.
In addition to the undesirable other optical aberrations produced by the peripheral portion of the device, decenterable vision correction devices that employ the myopia treatment method as taught by U.S. Pat. No. 7,025,460 can also suffer from blurring due to defocus during eccentric gaze. The repositioning of peripheral focal points for retarding the progression of myopia as taught by U.S. Pat. No. 7,025,460 implicitly introduces defocus to the peripheral image. While this is a desirable characteristic in terms of retardation of myopia progression when the eye is in the straight-ahead gaze, when the eye is in eccentric gaze, the image produced on the foveal region is produced by rays passing through a peripheral portion of the device and therefore will incur an amount of defocus. Thus, when the eye is directed to fixate on a peripheral visual object through a decenterable visual correction device, the image is blurred due to both defocus and other optical aberrations.
Such other optical aberrations (and also defocus in the case of decenterable vision correction devices during eccentric gaze) may be sufficiently, relatively, small in amount that vision remains acceptable to the wearer (who would be enjoying the benefit of producing a stimulus to retard or eliminate the progression of myopia and therefore may, in preference, be prepared to compromise on certain aspects of visual performance). However, other wearers may require certain select zone or multiple zones for which vision is critically important, and therefore a priority. Such zones, which will be called “vision priority zones” in this document, represent zones on a lens that correspond to particular visual directions (i.e. visual field direction or angles) for which the wearer requires good vision.
A few examples follow to illustrate when a wearer may choose to have a vision priority zone on their vision correction devices and where these associated vision priority zones may be located on the device.
In one example, a wearer may be engaged in driving a vehicle and requires not only good vision in the straight-ahead (on-axis) direction (as is provided by the method of U.S. Pat. No. 7,025,460) but also useful vision along a horizontal line representing a visual “sweep” of many visual objects (at many directions of gaze) lying on or across a roadway. Since the task of driving requires the wearer to primarily be visually aware along a horizontal line (e.g. to check for traffic in the cross street at a junction), the select vision priority zone for which useful vision is required would be represented by a band lying in a horizontal line. It should be noted that useful vision in the context of peripheral vision is a relative term since the density of photo-receptors on the retina decreases away from the fovea; hence there exists a physiological limit to visual acuity at the peripheral retina which decreases away from central, foveal vision.
In another example, the wearer may be engaged in a visual task which requires acute recognition and identification of fine visual objects in an extended central field (i.e. the region immediately surrounding the straight-ahead direction). Examples of such tasks may include radar operation for which the operator is required to view a radar screen or computer monitor and quickly detect and identify small points. For such wearers, the expanse (i.e. area or field of vision) of good on-axis and ‘nearly’ on-axis (a region peripheral to but near the central point also called the “para-central” region) visual acuity offered by the method according to U.S. Pat. No. 7,025,460 may be insufficient and a vision priority zone providing a wider para-central zone of good vision may be beneficial. The select zone of vision for this example may be a region approximately centered on the visual axis which subtends a field of view approximately equal to the size of the visual task (e.g. a computer monitor, a radar display unit, a musician's score, an artist's easel, an architect's drafting board, etc).
In yet another example, when a wearer of a decenterable vision correction device (such as a pair of spectacles) is reading, both eyes tend to point downward and converge (i.e. point slightly closer towards the direction of the nose). In this direction of gaze, each eye is looking through a point of the device that is positioned relatively lower and more “nasally” (a term used by eye-care practitioners to indicate a direction towards the nose—i.e. leftward for the right eye and rightward for the left eye) than the distance visual point. This point is called the “near visual point”. Thus for the case of a decenterable vision correction device, a wearer who is engaged for long periods in reading tasks (for example clerical work, book proof-reading, fine-arts such as engraving, embroidery) may require, in addition to good on-axis vision through the centre of the device, a select vision priority zone at the near visual point that provides good visual acuity. The useful size of the vision priority zone at the near visual point will depend on the size of the near work material (e.g. book page, artwork, etc). Given the foregoing, it would be desirable to provide further improvements in methods and visual treatment devices for the retardation or cessation of progression of myopia.